Device for pumping fluid from a wellbore

ABSTRACT

A pump device for pumping fluid from a wellbore, comprising: a piston member ( 38 ) movably arranged in a pump chamber ( 36 ) between a first position and a second position, in a manner allowing the piston member to perform a series of pump strokes, whereby at each pump stroke a volume of said fluid is pumped out of the pump chamber ( 36 ) by the piston member; a fluid conduit ( 20 ) arranged to move the piston member ( 38 ) from the first position to the second position by a selected fluid pressure increase in the fluid conduit; an inlet channel ( 60 ) connecting the pump chamber to a storage chamber ( 52 ) for receiving said volume of fluid from the pump chamber ( 36 ); first valve means ( 62 ) arranged in the inlet channel for allowing fluid flow from the pump chamber ( 36 ) to the storage chamber ( 52 ) and for preventing fluid flow from the storage chamber to the pump chamber; pressurising means ( 57 ) for applying hydraulic pressure to the volume of fluid in the storage chamber ( 52 ); an outlet channel ( 64 ) for discharging the volume of fluid from the storage chamber ( 52 ) into the fluid conduit ( 20 ); second valve means ( 68 ) for allowing fluid flow from the storage chamber ( 52 ) to the pump chamber ( 36 ) and for preventing fluid flow from the pump chamber to the fluid conduit ( 20 ); a fluid inlet ( 46 ) for inflow of said volume of fluid from the wellbore into the pump chamber ( 36 ); and third valve means ( 50 ) to prevent outflow of fluid from the pump chamber ( 36 ) to the fluid inlet ( 46 ).

The present invention relates to a device and a method for pumping fluidfrom a wellbore formed in an earth formation.

In hydrocarbon wells, such as gas wells, it frequently occurs thatwater, for instance from the surrounding earth formation or condensedwater, accumulates in the wellbore. Part of the formation water orcondensed water generally flows out of the wellbore together with theproduced gas. However, over time a significant amount of water mayaccumulate in the lower portion of the wellbore. Other liquids, such asgas condensate, may also accumulate in the wellbore.

The accumulated liquid in the wellbore is potentially detrimental to theproduction of hydrocarbon gas from the wellbore. Therefore, generallythere is a need to remove the accumulated liquid from the wellbore usinga technique referred to as deliquefaction.

US-2010/0051282-A1 discloses a pump arranged in a wellbore, which pumpis driven by a power fluid that is pumped through an inflow conduitpassing through a production tubing of the wellbore. The formation waterpumped by the pump is mixed with the power fluid and discharged from thewellbore via a fluid return line passing through the production tubing.

In view of the limited diameter of a wellbore and the correspondinglylimited available space, it is a drawback of the known pump system thatseparate fluid conduits are needed in the wellbore for driving thedownhole pump and for discharging the pumped water to surface.

WO-86/02412 discloses a surface powered pump unit for pumping oil orwater from wells. The pump unit comprises a downhole pump connected tosurface via a conventional tubing string.

It is an object of the invention to provide an improved device forpumping fluid from a wellbore.

In accordance with the invention there is provided a device for pumpingfluid from a wellbore formed in an earth formation, comprising:

-   -   a piston member arranged in a pump chamber in a manner allowing        the piston member to perform a series of pump strokes whereby at        each pump stroke a volume of said fluid is pumped out of the        pump chamber by the piston member;    -   a fluid conduit arranged so as to induce each pump stroke of the        piston member by a selected fluid pressure increase in the fluid        conduit;    -   a storage chamber for receiving said volume of fluid from the        pump chamber and pressurising means for applying hydraulic        pressure to the volume of fluid in the storage chamber;    -   an outlet channel for discharging the volume of fluid from the        storage chamber into the fluid conduit; and    -   closure means for closing the outlet channel during each pump        stroke of the piston member.

The invention also relates to a method of pumping fluid from a wellboreformed in an earth formation, the method comprising:

-   -   providing a piston member arranged in a pump chamber in a manner        allowing the piston member to perform a series of pump strokes        whereby at each pump stroke a volume of said fluid is pumped out        of the pump chamber by the piston member;    -   applying a selected fluid pressure increase in a fluid conduit        arranged so as to induce each pump stroke of the piston member        by the selected fluid pressure increase;    -   receiving said volume of fluid from the pump chamber in a        storage chamber and applying hydraulic pressure to the volume of        fluid in the storage chamber using pressurising means;    -   discharging the volume of fluid from the storage chamber into        the fluid conduit via an outlet channel; and    -   closing the outlet channel during each pump stroke of the piston        member using closing means.

With the device and the method of the invention it is achieved that thefluid conduit serves both for driving the device to perform the pumpcycles, and for discharging fluid that is pumped out of the wellbore.The ability to use a single fluid conduit for these two functionalitiesobviates the need to install and operate separate fluid conduits fordriving the downhole pump and for discharging the pumped fluid, as isthe case in the prior art.

Suitably the device comprises an inlet for inflow of said volume offluid from the wellbore into the pump chamber, the inlet being providedwith first valve means preventing outflow of fluid from the pump chambervia the inlet.

To prevent backflow of pumped fluid into the pump chamber, the pumpchamber suitably is in fluid communication with the storage chamber viaa transfer channel provided with second valve means preventing flow offluid from the storage chamber to the pump chamber via the transferchannel.

To prevent fluid from flowing directly from the fluid conduit to thestorage chamber, preferably the outlet channel is provided with thirdvalve means preventing flow of fluid from the fluid conduit to thestorage chamber via the outlet channel.

Suitably the pressurising means includes a reciprocating member arrangedin the storage chamber and means for exerting a force to thereciprocating member so as to press the reciprocating member against thevolume of fluid in the storage chamber. For example, said means forexerting a force to the reciprocating member comprises a volume ofcompressed gas, such as Nitrogen gas.

In an advantageous embodiment of the device, the outlet channeldebouches into the pump chamber, and the closure means comprises a wallportion of the piston member that closes the outlet channel during eachpump stroke of the piston member.

The volume of fluid may be efficiently discharged from the storagechamber if the wall of the piston member is provided with a passage thatprovides fluid communication between the outlet channel and the interiorof the conduit in between successive pump strokes of the piston member.

In each pump stroke the piston member moves from a first end position toa second end position. Advantageously said passage provides fluidcommunication between the outlet opening and the interior of the fluidconduit when the piston member is in the first end position.

In a practical application, the wellbore is provided with a productiontubing for transporting produced hydrocarbon fluid to surface, whereinthe pump device is positioned in a lower portion of the productiontubing, and wherein the fluid conduit extends through the productiontubing to surface.

The device is suitably arranged to pump water from a body of water viathe fluid conduit to surface, which body of water has entered thewellbore from the surrounding earth formation.

Suitably the pump device or a part thereof is retrievably connected tothe conduit so as to allow the pump device or said part thereof to beretrieved through the fluid conduit to surface by means of a wire line.This may be done, for example, for maintenance purposes.

The piston member generally performs a series of return strokesalternatingly with the series of pump strokes, whereby each returnstroke suitably is induced by a spring that is compressed during eachpump stroke by virtue of said selected fluid pressure increase in thefluid conduit.

The invention will explained hereinafter in more detail and by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 schematically shows an exemplary wellbore provided with thedevice for pumping fluid from the wellbore in accordance with theinvention;

FIG. 2 schematically shows a first embodiment of a pump assembly of thedevice for pumping fluid from the wellbore;

FIG. 3 schematically shows a second embodiment of a pump assembly of thedevice for pumping fluid from the wellbore;

FIG. 4 schematically shows a third embodiment of a pump assembly of thedevice for pumping fluid from the wellbore; and

FIG. 5 shows the third embodiment with some components disassembled.

In the Figures and the detailed description hereinafter, like referencesigns relate to like components.

Referring to FIG. 1 there is shown a wellbore 1 that has been drilledfrom surface 3 through an earth formation 4 to a reservoir layer 6 ofthe earth formation. The reservoir 6 comprises hydrocarbon gas and maybe located at a depth of, for example, 3000 m. The wellbore 1 is linedwith casings 8A-C and a liner 8D arranged in a conventional nestedarrangement. The casings 8A-C extend from a wellhead 9 at surface intothe wellbore 1. A Christmas tree 10 is provided on top of the wellhead9. The liner 8D is suspended from casing 8C by a liner hanger 12arranged at a down hole location, and extends from there to thereservoir layer 6. A lower portion 14 of the liner 8D is provided withperforations 15 to allow hydrocarbon fluid from the reservoir layer 6 toenter the lower portion 14 of the liner.

A production tubing 16 extends from wellhead 9 and Christmas tree 10through the interior of casing 8C and liner 8D, into the lower linerportion 14. The production tubing 16 is internally provided with asub-surface safety valve (sssv) 17 that is controlled by a hydrauliccontrol line (not shown) extending from surface 3 into the wellbore 1along the outside of the production tubing 16. The sub-surface safetyvalve 17 is located at a depth of, for example, approximately 100 m, andis adapted to close the production tubing in the event of an emergency.A production packer 18 is provided between the production tubing 16 andthe liner 8D to seal a lower portion of the wellbore 1 where the inletof the production tubing 16 is located, from an upper portion of thewellbore 1. The wellbore 1 contains a body of water 19 that has enteredthe wellbore from the surrounding earth formation.

A fluid conduit in the form of a coiled tubing 20 extends from thewellhead 9 at surface through the interior of the production tubing 16,to near the open lower end of the production tubing 16. The coiledtubing 20 is supplied from a large reel (not shown) and may have anouter diameter of, for example, 2.5-8.5 cm. At the lower end of thecoiled tubing 20 there is provided a pump assembly 22 for pumping waterfrom the body of water 19 out of the wellbore 1 via the coiled tubing.The coiled tubing 20 is fluidly connected to an outlet conduit 24provided at the Christmas tree 10, and to a surface pump 26 forselectively increasing the fluid pressure in the coiled tubing 20 bypumping water into the coiled tubing. The outlet conduit 24 is providedwith a valve (not shown) that closes the outlet conduit when the surfacepump 26 is actuated. The production tubing 16 is at surface fluidlyconnected to a flow line 28 for transporting hydrocarbon gas that isproduced from the reservoir layer 6 via the production tubing 16.

Referring further to FIG. 2 there is shown a schematic representation ofa first embodiment 29 of the pump assembly 22. The first embodiment 29comprises a cylindrical housing 30 positioned inside the coiled tubing20 at the lower end thereof and fixedly connected to the coiled tubingby a lock ring 32. A seal ring 34 seals the housing 30 relative to thewall of the coiled tubing.

An upper part of the housing 30 is provided with a cylindrical pumpchamber 36 in which a piston member 38 is arranged. The piston member 38is axially movable in the pump chamber 36 between an upper position anda lower position, and is provided with annular seals 40 a, 40 b to sealthe piston member to the wall of the pump chamber. The wall of the pumpchamber 36 is provided with an annular seal 40 c for additional sealing.A compression spring 41 is provided between a flanged upper end 42 ofthe piston member 38 and the housing 30 so as to bias the piston member38 against a stop ring 43 at the inner surface of the coiled tubing 20.When the flanged upper end 42 is biased against the stop ring 43, thepiston member 38 is in the upper position. The piston member 38 has atubular portion 44 which, at its upper end, is in fluid communicationwith the interior of the coiled tubing 20. The wall of the tubularportion 44 is provided with a radial passage 45 located between theannular seals 40 a, 40 b. The housing 30 has a fluid inlet 46 that is influid communication with the body of water 19 via inlet openings 48provided in the wall of the coiled tubing 20. A check valve 50 isarranged between the fluid inlet 46 and the pump chamber 36 to allowflow of fluid from the fluid inlet 46 to the pump chamber 36, and toprevent flow of fluid from the pump chamber 36 to the fluid inlet 46.

A lower part of the housing 30 is provided with a cylindrical storagechamber 52 in which a reciprocating member 54 is arranged, thereciprocating member 54 being freely movable in axial direction in thestorage chamber 52 between an upper position and a lower position. Anannular seal 55 is provided between the reciprocating member 54 and thewall of the storage chamber 52. The storage chamber 52 is at the lowerend thereof closed by means of a plug 56. The portion of the storagechamber 52 that is located between the reciprocating member 54 and theplug 56 is filled with compressed Nitrogen gas, and is referred to asthe gas chamber 57. The plug 56 is provided with a pressure gauge 58 formeasuring the pressure of the Nitrogen gas.

A transfer channel 60 is formed in the housing 30, which transferchannel provides fluid communication between the pump chamber 36 and thestorage chamber 52. A first check valve 62 allows flow of fluid from thepump chamber 36 to the storage chamber 52 via the transfer channel 60and prevents flow of fluid through the transfer channel in reversedirection.

The storage chamber 52 has an outlet channel 64 formed in the housing30, which outlet channel is at its upper end aligned with the passage 45in the wall of the piston member 38 when the piston member is in theupper position. Thus, when the piston member 38 is in the upperposition, the outlet channel 64 provides fluid communication between thestorage chamber 52 and the coiled tubing 20. When the passage 45 is notaligned with the outlet channel 64, that is, when the piston membermoves toward its lower position or moves from the lower position to theupper position, the outlet channel 64 is closed by a wall portion 66 ofthe piston member so that flow of fluid from the storage chamber 52 tothe coiled tubing 20 is blocked. Further, the outlet channel 64 isprovided with a second check valve 68 that allows flow of fluid from thestorage chamber 52 to the coiled tubing 20 and prevents flow of fluidfrom the coiled tubing 20 to the storage chamber 52.

Referring further to FIG. 3 there is shown a second embodiment 70 of thepump assembly 22. The second embodiment 70 of the pump assembly islargely similar to the first embodiment 29 of the pump assembly, howeverdiffering therefrom in the following aspects. The second embodiment 70includes a housing 72 connected to the lower end of the coiled tubing20, for example by a threaded connection. The housing 72 includes lowerand upper housing sections 72 a, 72 b which are interconnected by athreaded connection 73.

An insert member 74 extends into an upper end of housing section 72 aand into a lower end of housing section 72 b. Annular spaces 78 a to 78d are provided between the insert member 74 on one hand and the housingsections 72 a, 72 b on the other hand. The annular spaces 78 a-d aresealed from each other and from the pump chamber 36 by appropriateannular seals (not shown). The insert member 74 has a fluid inlet 82 influid communication with the body of water 19 via annular space 78 c andan inlet opening 84 in housing section 72 a. The first check valve 62 isprovided to allow flow of fluid from the fluid inlet 82 to the pumpchamber 36, and to prevent flow of fluid from the pump chamber to thefluid inlet.

The storage chamber 52 has an outlet channel 64 formed by a bore 88 inthe lower housing section 72 a, a bore 89 in the insert member, and abore 90 in the upper housing section 72 b. The tubular portion 44 of thepiston member 38 is provided with a series of radial passages 45 locatedbetween the annular seals 40 a, 40 b.

The bore 90 is at its upper end aligned with the passages 45 in the wallof the piston member 38 when the piston member is in the upper position.Thus, in this position the outlet channel 64 provides fluidcommunication between the storage chamber 52 and the coiled tubing 20.When the passages 45 are not aligned with the bore 90, that is, when thepiston member moves toward its lower position or moves from the lowerposition to the upper position, the outlet channel 64 is closed by thewall portion 66 of the piston member so that flow of fluid from thestorage chamber 52 to the coiled tubing 20 is blocked. The second checkvalve 68 allows flow of fluid from the storage chamber 52 to the coiledtubing 20 via the outlet channel 64 and prevents flow of fluid throughthe outlet channel in reverse direction.

Referring further to FIGS. 4 and 5 there is shown a third embodiment 92of the pump assembly 22. The third embodiment 92 of the pump assembly islargely similar to the second embodiment 70 of the pump assembly,however differing therefrom in the following aspects. The thirdembodiment 92 comprises a housing 94 connected to the lower end of thecoiled tubing 20, for example by a threaded connection. The housing 94includes lower and upper housing sections 94 a, 94 b which areinterconnected e.g. by a threaded connection. A tube 95 extends aroundthe upper housing section 94 b whereby an annular space 96 is definedbetween the tube 95 and the upper housing section 94 b. The tube 95 isat both ends thereof sealed to the upper housing section 94 b bysuitable seals (not shown).

An insert member 97 extends into an upper end of housing section 94 aand into a lower end of housing section 94 b. The insert member 97 isretrievable from the housing 94 by means of a wireline (not shown) thatmay be lowered from surface into the wellbore. The wireline isconnectable into a tubular upper portion 98 of the insert member 97,which upper portion is latched to housing section 94 b by a latchingmechanism 99. A lower portion 100 of the insert member is axiallyadjustable relative to the upper portion 98. Annular spaces 102 a-c areprovided between the lower portion 100 on one hand and the housingsections 94 a, 94 b on the other hand. The annular spaces 102 a-c aresealed from each other and from the pump chamber 36 by appropriateannular seals (not shown). The insert member 97 is provided with a fluidinlet 104 that is in fluid communication with the body of water 19 viaan inlet opening 106 in housing section 94 a. A check valve 108 isprovided to open or close the fluid communication between the conduit 20and the fluid inlet 104 and inlet opening 106.

The pump chamber 36 is in fluid communication with the storage chamber52. The fluid communication is for instance via a transfer channel 110formed in the tubular upper portion 98 of the insert member, a bore 112provided in the insert member, and a bore 114 provided in the lowerhousing section 94 a. The first check valve 62 allows flow of fluid fromthe pump chamber 36 to the storage chamber 52, but prevents flow offluid in reverse direction.

The piston member 38 also is retrievable from the housing 94 by awireline that is connectable into a tubular upper end 116 of the pistonmember. Thereto the piston member 38 is provided with a latching member118 axially slidable relative to the tubular portion 44 of the pistonmember. The latching member 118 has fingers 120 that latch into a recess121 at the inner surface of upper housing section 94 b. The latchingmember 118 is provided with annular seals 122, 123 to seal the latchingmember to the tubular portion 44 of the piston member, and to housingsection 94 b.

The storage chamber 52 has an outlet channel 124 formed by bore 88 inthe lower housing section 94 a, a bore 126 in the insert member, radialopening 111, annular space 96, and a series of radial bores 128 formedin upper housing section 94 b. The radial bores 128 are aligned with thepassages 45 in the wall of the piston member 38 when the piston memberis in the upper position. Thus, in this position the outlet channel 124provides fluid communication between the storage chamber 52 and thecoiled tubing 20. When the passages 45 are not aligned with the radialbores 128, that is, when the piston member moves toward its lowerposition or moves from the lower position to the upper position, theoutlet channel 124 is closed by the wall portion 66 of the piston memberso that flow of fluid from the storage chamber 52 to the coiled tubing20 is blocked. The second check valve 68 allows flow of fluid from thestorage chamber 52 to the coiled tubing 20, but prevents flow of fluidin reverse direction.

During normal use of the first embodiment 29 of the pump assembly, thepump chamber initially contains a volume of water and the coiled tubing20 is filled with water. A pump cycle of the pump assembly is started byactuating the surface pump 26 so as to induce a pressure increase in thewater column in the coiled tubing 20. The magnitude of the pressureincrease is selected such as to cause the piston member 38 to move fromits upper position to its lower position thereby performing a pumpstroke whereby the volume of water is pumped out of the pump chamber 36and into the storage chamber 52 via the transfer channel 60. As a resultthe reciprocating member 54 moves downwardly and compresses the Nitrogengas in the gas chamber 57. During the pump stroke, outflow of water fromthe storage chamber 52 via the outlet channel 64 is prevented since theoutlet channel is blocked at its upper end by wall portion 66 of thepiston member 38.

Upon arrival of the piston member 38 at its lower position, actuation ofthe surface pump 26 is stopped so that the fluid pressure in the coiledtubing 20 drops. As a result, the compression spring 41 causes thepiston member 38 to move back to its upper position. The piston member38 thereby draws a further volume of water from the wellbore 1 into thepump chamber 36 via the fluid inlet 46. As the piston member 38 arrivesat its upper position, the radial passage 45 of the piston member 38becomes aligned with the outlet channel 64. The outlet channel 64 isthereby no longer blocked by the wall portion 66 of the piston member,and the compressed Nitrogen gas in the gas chamber 57 causes thereciprocating member 54 to move upwardly thereby pumping the volume ofwater via the outlet channel 64 into the tubular portion 44 of thepiston member 38 and thence further into the coiled tubing 20. Fromthere, the volume of water flows is discharged via the outlet conduit 24at surface. A next pump cycle is then started in similar manner wherebysaid further volume of water is pumped out of the wellbore, etc.

Normal use of the second embodiment 70 of the pump assembly issubstantially similar to normal use of the first embodiment 29. Duringeach pump cycle, the volume of water is pumped from the pump chamber 36to the storage chamber 52 via the transfer channel 60, and from thestorage chamber 52 to the coiled tubing 20 via the outlet channel 64. Incase the pump assembly 70 needs to be disassembled, e.g. for maintenancepurpose, the housing sections 72 a, 72 b are disconnected from eachother at the threaded connection 73. Thereafter, the insert member 74can be removed from the housing sections 72 a, 72 b.

Normal use of the third embodiment 92 of the pump assembly issubstantially similar to normal use of the first embodiment 29. Duringeach pump cycle, the volume of water is pumped from the pump chamber 36to the storage chamber 52 via the transfer channel 110, and from thestorage chamber 52 to the coiled tubing 20 via the outlet channel 124.Should it be required to retrieve the piston member 38 to surface, e.g.for maintenance purpose, a wireline is lowered into the coiled tubing 20and connected to the tubular upper end 116 of the piston member. Thewireline is then pulled back whereupon the fingers 120 of the latchingmember 118 unlatch from the recess 121 in the housing section 94 b, andthe piston member 38 together with the latching member 118 is retrievedto surface through the coiled tubing.

After the piston member 38 has been retrieved to surface, the insertmember 97 also may be retrieved by means of the wireline that is loweredinto the coiled tubing 20 and connected to the tubular upper portion 98of the insert member 97. The wireline is then pulled back whereupon thelatching mechanism 99 unlatches, and the insert member 97 is retrievedto surface through the coiled tubing.

In a variation of the detailed embodiments described above, a conductorcable extends from surface to the downhole pump assembly through theinterior of the coiled tubing or outside the coiled tubing. Theconductor cable may be used for transmitting downhole data to surface,for example data on the water level in the wellbore, downholetemperature, or downhole pressure.

The description above describes exemplary embodiments of the presentinvention, wherein various modifications are conceivable within thescope of the appended claims. Features of respective embodiments may forinstance be combined.

1. A pump device for pumping fluid from a wellbore, comprising: a pistonmember movably arranged in a pump chamber between a first position and asecond position, in a manner allowing the piston member to perform aseries of pump strokes, whereby at each pump stroke a volume of saidfluid is pumped out of the pump chamber by the piston member; a fluidconduit arranged to move the piston member from the first position tothe second position by a selected fluid pressure increase in the fluidconduit; an inlet channel connecting the pump chamber to a storagechamber for receiving said volume of fluid from the pump chamber; firstvalve means arranged in the inlet channel for allowing fluid flow fromthe pump chamber to the storage chamber and for preventing fluid flowfrom the storage chamber to the pump chamber; pressurising means forapplying hydraulic pressure to the volume of fluid in the storagechamber; an outlet channel for discharging the volume of fluid from thestorage chamber into the fluid conduit; second valve means for allowingfluid flow from the storage chamber to the pump chamber and forpreventing fluid flow from the pump chamber to the fluid conduit; afluid inlet for inflow of said volume of fluid from the wellbore intothe pump chamber; and third valve means to prevent outflow of fluid fromthe pump chamber to the fluid inlet.
 2. The device of claim 1, whereinthe pressurising means include a reciprocating member arranged in thestorage chamber and force means for exerting a force to thereciprocating member to press the reciprocating member against thevolume of fluid in the storage chamber.
 3. The device of claim 2,wherein said force means comprise a volume of compressed gas.
 4. Thedevice of claim 3, wherein said volume of compressed gas comprisesNitrogen gas.
 5. The device of claim 1, wherein the second valve meanscomprise an opening in a wall of the piston member.
 6. The device ofclaim 5, wherein the opening provides fluid communication between theoutlet channel and the interior of the conduit in between successivepump strokes of the piston member for opening the outlet channel whenthe opening is aligned with the outlet channel, and for closing theoutlet channel when the opening is not aligned with the outlet channel.7. The device of claim 6, wherein the opening provides fluidcommunication between the outlet channel and the interior of the fluidconduit when the piston member is in the first position.
 8. The deviceof claim 1, wherein the wellbore is provided with a production tubingfor transporting produced hydrocarbon fluid to surface, wherein the pumpdevice is positioned in a lower portion of the production tubing, andwherein the fluid conduit extends through the production tubing tosurface.
 9. The device of claim 1, wherein the wellbore contains a bodyof water that has entered the wellbore from the surrounding earthformation, and wherein the device is arranged to pump water from thebody of water via the fluid conduit to surface.
 10. The device of claim1, wherein the pump device or a part thereof is retrievably connected tothe conduit so as to allow the pump device or said part thereof to beretrieved through the fluid conduit to surface by means of a wire line.11. The device of claim 1, wherein the piston member is arranged toperform a series of return strokes alternatingly with the series of pumpstrokes, and wherein the device further comprises a spring for inducingeach return stroke, the spring being adapted to be compressed duringeach pump stroke by said selected fluid pressure increase in the fluidconduit.
 12. A method of pumping fluid from a wellbore formed in anearth formation, the method comprising: providing a piston membermovably arranged in a pump chamber between a first position and a secondposition, in a manner allowing the piston member to perform a series ofpump strokes, whereby at each pump stroke a volume of said fluid ispumped out of the pump chamber by the piston member; applying a selectedfluid pressure increase in a fluid conduit to move the piston memberfrom the first position to the second position to induce a pump stroke;receiving said volume of fluid from the pump chamber in a storagechamber via an inlet channel; arranging first valve means in the inletchannel for allowing fluid flow from the pump chamber to the storagechamber and for preventing fluid flow from the storage chamber to thepump chamber; applying hydraulic pressure to the volume of fluid in thestorage chamber using pressurising means; discharging the volume offluid from the storage chamber into the fluid conduit via an outletchannel; and arranging second valve means in the outlet channel forallowing fluid flow from the storage chamber to the pump chamber and forpreventing fluid flow from the pump chamber to the fluid conduit; andarranging third valve means for allowing inflow of said volume of fluidfrom the wellbore via a fluid inlet into the pump chamber, and toprevent outflow of fluid from the pump chamber to the fluid inlet. 13.The method of claim 12, wherein the piston member performs a series ofreturn strokes alternating with the series of pump strokes, wherein eachreturn stroke is induced by a spring force applied to the piston memberusing a spring, and wherein the spring is compressed during each pumpstroke by application of said selected fluid pressure increase in thefluid conduit